1. Field of the Invention
The present invention is related to the field of acoustic array wellbore logging instruments. More specifically, the present invention is related to methods of processing receiver signals from an acoustic array wellbore logging instrument in order to determine acoustic energy transmission properties of earth formations penetrated by a wellbore.
2. Discussion of the Related Art
Acoustic array wellbore logging tools are known in the art for determining acoustic properties of earth formations penetrated by a wellbore. The array acoustic wellbore logging tools known in the art typically include an energy emitting transducer, called a transmitter, and a plurality of receiving transducers, called receivers, axially spaced apart from the transmitter along an elongated tool mandrel. The tool is typically lowered into the wellbore at one end of an armored electrical cable. The transmitter periodically energizes the wellbore with pulses of acoustic energy. The acoustic energy travels through a fluid filling the wellbore and interacts with the interface between the fluid and the wall of the wellbore. Some of the acoustic energy can then travel along the wellbore wall. After travelling along the wall of the wellbore, some of the energy can travel back into the wellbore towards the tool where it can be detected by the receivers. The receivers convert the acoustic energy into electrical signals having amplitudes corresponding to the acoustic energy amplitude.
Tools known in the an can digitize the receiver signals in an electronics unit disposed within the tool mandrel, and can transmit the digitized signals along the electrical cable to processing and recording equipment located at the earth's surface. The properties of interest of various earth formations traversed by the logging tool can be determined by performing various calculations on the digitized signals.
For example, a method which is known in the an for determining acoustic transmission velocity of the formation, the method being called semblance correlation, comprises determining values of time difference between the signals from each of the receivers at which the degree of correspondence between the signals reaches a maximum. The time difference is then used to calculate a formation acoustic velocity since the distances between the receivers are known.
Other methods known in the art for determining formation velocity from the receiver signals include so-called N-th root stacking, which includes digitally summing or stacking predetermined portions of the receiver signals in order to improve the signal-to-noise ratio of the velocity determinations.
A drawback to the methods known in the art for determining formation velocity from receiver signals of an array acoustic logging tool is that the receiver signals can include responses to more than one mode of acoustic energy propagation. Propagation modes are related to the manner in which the particles of the formation move relative to the direction of propagation of the acoustic energy and can include so-called compressional mode, shear or flexural mode and Stoneley modes. Certain acoustic energy propagation modes can have significant overlap in time, making isolation of the desired mode difficult using N-th root stacking or semblance correlation.
It is also known in the art to distinguish one propagation mode form another by filtering the receiver signals with a frequency bandpass filter having a passband corresponding to the frequency content of the desired propagation mode. A drawback to bandpass filtering is that the desired propagation mode may have significant overlap in frequency content with other propagation modes, making difficult the isolation of the desired mode, and subsequent determination of the velocity of that mode. For example, shear propagation mode, in which the acoustic energy propagates substantially perpendicularly to the direction of particle motion, can have different acoustic velocities for different orientations of particle motion in some anisotropic earth formations. A single acoustic signal comprising the two different orientations of shear waves, representing shear propagation at two different velocities, would be difficult to separate into its component modes using bandpass filtering and semblance correlation for determining acoustic velocity, since the frequency content and time span of the two modes would have substantial overlap.
Accordingly, it is an object of the present invention to provide a method of determining the velocity of an acoustic signal propagating in a desired propagation mode from receiver signals of an acoustic array logging tool, which provides improved performance in the event of signals from other propagation modes overlapping the desired mode in time and frequency content.